1. Field of the Invention
The present invention relates to an electron emission display device. More particularly, the present invention relates to an electron emission display device having alignment marks for aligning two substrates facing each other to form a vacuum envelope.
2. Description of the Related Art
Generally, electron emission display devices are classified into a first type using a hot cathode as an electron emission source and a second type using a cold cathode as the electron emission source. Cold cathode electron emission display devices include a metal-insulator-metal (MIM) type, a metal-insulator-semiconductor (MIS) type, a surface conduction emission (SCE) type and a field emitter array (FEA) type.
The MIM type and the MIS type electron emission display devices have electron emission regions with a metal/insulator/metal (MIM) structure and a metal/insulator/semiconductor (MIS) structure, respectively. When voltages are applied to the two metals, or the metal and the semiconductor, on respective sides of the insulator, electrons supplied by the metal or semiconductor on the lower side pass through the insulator due to a tunneling effect and arrive at the metal on the upper side. Of the electrons that arrive at the metal on the upper side, those that have energy greater than or equal to the work function of the metal on the upper side are emitted from an upper electrode.
The SCE type electron emission display device includes first and second electrodes arranged on a substrate parallel to each other, and a conductive film disposed between the first and the second electrodes. Micro-cracks are made in the conductive film to form electron emission regions. When voltages are applied to the first and second electrodes while making an electric current flow to the surface of the conductive film, electrons are emitted from the electron emission regions.
The FEA type electron emission display device is based on the principle that when a material having a low work function or a high aspect ratio is used as an electron emission source, electrons are easily emitted from the material with the application of an electric field thereto under a vacuum atmosphere. A front sharp-pointed tip structure based on, e.g., molybdenum (Mo), silicon (Si) or carbonaceous materials, has been developed to form electron emission regions.
Although the specific structures of the electron emission display devices using the cold cathode are differentiated depending upon the types thereof, they basically have first and second substrates forming a vacuum envelope, and electron emission regions formed on the first substrate together with driving electrodes for controlling the emission of electrons from the electron emission regions. Phosphor layers may be formed on the second substrate for forming an image. An anode electrode may be provided on the second substrate for accelerating the electrons emitted from the first substrate toward the phosphor layers.
In the above-structured electron emission display device, cross-shaped alignment keys may be formed at the peripheries of the two substrates, and the two substrates may be aligned to each other based on the alignment keys.
However, when the two substrates are aligned to each other using the alignment keys, there may be a large distance between the active area where pixels are actually arranged and the alignment keys. If the distance is varied in any way, the electron emission regions and the phosphor layers within the active area may be displaced from each other even though the alignment keys are aligned with each other.
In such a case, the worker cannot check whether the alignment within the active area is made correctly. When the alignment within the active area is not made correctly, the light emission and the display operation of the electron emission display device may be problematic.